Conventionally, a vehicle running test device that simulates a running state of a vehicle, such as an automobile, running on the actual ground is used to evaluate running performance, fuel consumption performance, and exhaust characteristics of the vehicle.
The vehicle running test device includes columnar rollers and chains. Each roller has an outer circumferential surface on which a wheel of a vehicle is mounted and is rotated as the wheel rotates. The chains are connected to the vehicle and restrict displacement of the vehicle from the rollers by pulling the vehicle toward the rollers. The vehicle running test device calculates running resistance of the vehicle on the actual ground using the speed of the vehicle running on the rollers, which is obtained based on rotation of the rollers. The device then applies torque corresponding to the running resistance to the rollers. As the vehicle runs on the rollers in accordance with a prescribed running pattern, the torque, which acts on the rollers in correspondence with the running resistance, changes in correspondence with change of the running resistance applied to the vehicle when the vehicle runs on the actual ground in accordance with the running pattern.
A running test of a vehicle, which is carried out to, for example, evaluate the fuel consumption performance of the vehicle, is performed in the following manner. A vehicle is first mounted on rollers of a vehicle running test device. Chains are then attached to the vehicle to stop the vehicle from displacing from the rollers. In this state, the vehicle is caused to run in accordance with a running pattern prescribed to evaluate the fuel consumption performance of the vehicle, which is, for example, the Japanese 10-15 mode cycle. Meanwhile, the rollers of the test device receive torque corresponding to the running resistance applied to the vehicle when the vehicle runs in accordance with the Japanese 10-15 mode cycle. Then, the fuel consumption of the vehicle running in the Japanese 10-15 mode cycle is measured. The fuel consumption performance of the vehicle is thus evaluated based on the measured values.
Specifically, when the vehicle runs on the actual ground, the running resistance increases as the running speed of the vehicle increases. However, the running resistance applied to the vehicle when the vehicle runs on the actual ground is also influenced by the friction coefficient between the wheels of the vehicle and the ground surface, in addition to the vehicle running speed. Thus, in the technique described in Patent Document 1, in order to apply the torque corresponding to the running resistance on the actual ground to the rollers, the running resistance is calculated with not only the vehicle running speed but also the friction coefficient between the wheels and the ground surface taken into consideration. Such calculation increases the accuracy of the obtained running resistance.
More specifically, the relationship between the slip ratio of each wheel with respect to the surface of the actual ground surface and the friction coefficient μj between the wheel and the actual ground surface is defined in advance using a map or a model expression. Then, the slip ratio of the wheel with respect to the roller of the vehicle running test device is determined. Based on the slip ratio, the friction coefficient μj is calculated using the map or the model expression. Further, the running resistance applied to the vehicle on the actual ground is calculated with the friction coefficient μj taken into consideration. The torque corresponding to the calculated running resistance is then applied to the roller.
However, in the running test of the vehicle using the test device, the drive force produced by the vehicle is different from the drive force generated by the vehicle running on the actual ground. As has been discovered, such difference in drive force decreases the accuracy of the test result.
Specifically, the contact state of each wheel with respect to the roller of the test device is different from the contact state of the wheel with respect to the actual ground. As a result, the running resistance applied to the vehicle by the roller becomes different from the running resistance applied to the vehicle running on the actual ground. Such difference results in the difference between the drive force produced by the vehicle in the test and the drive force generated by the vehicle on the actual ground.
Further, the difference in the drive force between the vehicle in the test and the vehicle on the actual ground may be brought about also by the chains pulling the vehicle toward the rollers to restrict displacement of the vehicle from the rollers. That is, as the vehicle is pulled toward the rollers by the chains, the load applied to each roller by the corresponding wheel increases compared to the load applied to the actual ground by the wheel. Thus, when the torque corresponding to the running resistance is applied to each roller, the running resistance applied to the vehicle by the roller increases by an amount corresponding to the aforementioned increase amount of the load. As a result, the drive force produced by the vehicle in the running test, which runs in the running pattern such as the Japanese 10-15 mode cycle, becomes greater than the drive force generated by the vehicle when the vehicle runs on the actual ground.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2004-20401